I once called the HWV and they would get a O/W cab for about $50 as they wanted $79/day for a car (later it went down to $33/day). The bus service comes and goes depending on the HI state grants. I would not count on it for airport travel. I would say I certainly would want a car if I was staying there for 10 days. So any fantastic places to go and see especially on that nice Big Island (dry Kona, wet Hilo, active volcanoes, non-active volcanoes, macademia nut farms (nice fresh free samples too}, lovely biological gardens, coffee plantations etc etc). Cars are usually inexpensive in HI, often about $20/day. I always get a car. You can always turn it in early if you dont use it. BTW if you park the car yourself, it is free (ie dont use the valet).
MisterNice